When Google rolls out its mobile-first index, it will begin by indexing individual sites that are ready for it.

This was confirmed by John Mueller in a recent Google Webmaster Central office-hours hangout. Mueller went on to say sites may or may not be informed the mobile-first switch has changed for them. This presents a challenge when it comes to tracking analytics and ranking positions.

On the positive side, with Google rolling out the mobile-first index on a per-site basis, those that are prepared have a good chance of being indexed before others. Those that aren’t ready for the mobile-first switch will have to wait a bit longer to be indexed, Mueller says.

This can be seen a positive thing for both searchers and site owners. Searchers will see only the most mobile-friendly sites, as intended, while site owners who have put in work preparing for the mobile-first index will be rewarded with priority indexing.